Various transmission apparatus have been proposed over the years employing gears, clutches, chain drives, belts and the like. As applied to the problem of bicycle propulsion, the traditional mechanism for transmitting power from the rider to a motive wheel is a chain drive wherein a drive sprocket is attached to the pedals of a bicycle and a driven sprocket is attached to the rear wheel. A chain is engaged in the teeth of the sprockets such that rotation of the pedals in a first direction induces the rear wheel to turn. The driven sprocket is generally coupled to the rear wheel via a ratchet or one-way clutch such that the sprocket acts on the rear wheel in one direction of rotation only, i.e., the forward direction, but spins freely in the other direction. This one-way coupling of the drive sprocket to the motive or driven wheel permits the bicyclist to stop pedaling when coasting or braking. It has also become quite common for bicycles to incorporate gear change apparatus to select from a set of available gear ratios between the driven sprocket and the drive sprocket. This is normally accomplished by providing a nest of progressively larger driven sprockets at the rear wheel and a number of drive sprockets coupled to the pedals. Movable chain guides in conjunction with a chain tensioner allow the bicyclist to select which sprockets are utilized by positioning the drive chain over the correct sprocket pair via levers and cables.
While the traditional bicycle chain drive has served its purposes well over the years, it does have certain limitations of functionality and safety. For example, in order to reduce weight, chain drives are typically exposed, creating a safety risk to the rider in the form of entangling clothing in the chain and sprocket. Selectable ratio bicycle transmissions are prone to shifting to the wrong gear and to positioning the chain in an intermediate position between sprockets, leading to a surprising and dangerous slipping of the chain and the consequent unbalancing of the cyclist. Because the chain and sprocket require lubrication, they become a magnet for dust and dirt which impedes efficient power transmission, wears the chain and sprockets and frequently ruins clothing that contacts it by imparting a dirty, greasy stain.
It is therefore an object of the present invention to provide a transmission that reliably and efficiently transmits power from a first mechanical movement to a second. It is a further an object to provide a transmission which can be applied to a bicycle that is cleaner, safer and easier to use than a chain drive.